Supporting a wind-driven electric generator

ABSTRACT

Among other things, a nacelle of a wind-driven electric generator is supported by a structure that includes at least three legs that define a lower tower that is relatively broad at its lower end and rises to a relatively narrow waist, and a nacelle support that is relatively narrow at its lower end where it is attached to the waist and relatively broader at its upper end where it supports the nacelle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/043,333, filed Apr. 8, 2008 and U.S. Provisional Application No. 61/043,327, filed Apr. 8, 2008, the entire disclosures of which are incorporated by reference herein.

BACKGROUND

This description relates to a supporting structure for a wind-driven electric generator.

Wind turbines, for example, traditionally have been mounted on lattice or monopole towers with or without guy lines.

To achieve higher power output (multi-megawatts, for example), some wind turbine designs use larger and heavier rotors (and corresponding nacelles) and place the rotors higher above the ground to access better wind. These features also result in higher maximum thrust loads (which increase with effective surface area of the rotor and nacelle) and larger moments tending to overturn the wind tower at the stationary base of the tower.

For a variety of reasons, a typical support tower for a high-power wind turbine is fabricated in sections that can be assembled as a tall, relatively slim, tapered cylindrical monopole. The tubular sections have diameters that are small enough to permit the sections, when resting on their sides, to be trucked under highway overpasses to the site. There, they are assembled to form the monopole, which is then lifted into position by a crane and mounted on a stationary foundation.

As an example, a two megawatt turbine using an 80-meter diameter rotor may be mounted on a monopole that has a maximum diameter of five meters at the bottom of the tower and tapers to a diameter of four meters at the top.

Several considerations are important in designing a tall, slim monopole tower. The relationship of the first frequency of its resonant bending to the frequency at which the rotor blades will pass the tower must be studied in order to avoid destructive tower resonance and catastrophic failure. In determining the height of the rotor hub, the benefit of capturing higher wind power and reducing wind shear, turbulence, and blade fatigue at greater heights must be balanced against the cost of a tall mobile crane or other device used for erecting the tower. Wind turbine and tower design must also accommodate the unloading and loading forces that each rotor blade experiences as it passes the tower.

The design must also reduce the chance of a rotor blade striking the tower as the blade flexes in the wind. For example, the nacelle may be lengthened to move the rotor plane further away from the tower. The rotor axis may be inclined away from horizontal. A narrowed waist can be provided on the monopole below the nacelle. The blades can be pre-bent or coned out of the rotor plane away from the tower. The tips of the blades can be hinged. The tower can be inclined at an angle to vertical. And the control system of the turbine may be designed to anticipate tower strikes.

In the field of power distribution lines, a support tower commonly has a lattice configuration similar to the design of the Eiffel Tower. A lattice tower requires less steel than a comparable non-lattice monopole tower and can be flared at the bottom of the tower to resist the overturning moment.

The appearance of a lattice tower is characterized by a high spatial frequency. It has been proposed to cover a lattice tower to make it look like a monopole for aesthetic reasons. Aesthetics can also relate to a choice of paint color for a blade of a wind turbine rotor.

SUMMARY

In general, in an aspect, a nacelle of a wind-driven electric generator is supported on a structure that includes at least three legs that define a lower tower that is relatively broad at its lower end and rises to a relatively narrow waist, and a nacelle support that is relatively narrow at its lower end where it is attached to the waist and relatively broader at its upper end where it supports the nacelle.

Implementations may include one or more of the following features.

Each of the legs of the lower tower comprises a tubular structure. Each leg has a cylindrical cross-section along most of its length. The ratio of the diameter to the thickness of each leg is between 50 and 170, e.g., between 60 and 100. Each of the legs comprises an assembly of separately fabricated sections along its length. Each of the separately fabricated sections is no larger than 4 meters in one dimension and no larger than 40 meters in a second dimension. The structure also includes one or more braces, e.g., horizontal or diagonal braces. There are no more than six (or no more than three, depending on aesthetic and structural considerations) horizontal braces between any pair of the legs. The braces are attached at regular intervals along a leg. At least one of the braces is attached to a leg at a point that is either about halfway or about one-third of the way along the length of the leg. The legs are arranged at equal angles around a vertical axis of the lower tower. The legs have the same lengths. The legs are oriented at the same angles to a vertical axis of the lower tower. Each of the legs is attached at its lower end to a stationary foundation. Each of the legs is straight. Each of the legs is curved.

The nacelle support has at least three legs. Each of the legs of the nacelle support is tubular. Each of the legs of the nacelle support has a cylindrical cross-section along most of its length. The ratio of the outside diameter of the each of the legs of the nacelle support to the thickness of the leg is between 50 and 170, e.g., between 60 and 100. Each of the legs of the nacelle support comprises an assembly of separately fabricated sections along its length. Each of the separately fabricated sections is no larger than 4 meters in one dimension and no larger than 40 meters in a second dimension. The nacelle support also includes one or more braces. The braces are horizontal or diagonal or both. There are no more than six horizontal braces between any pair of the legs of the nacelle support. There are no more than three horizontal braces between any pair of the legs of the nacelle support. The braces are attached to a leg of the nacelle support at regular intervals along the length of the leg. At least one of the braces is attached to the leg halfway or one-third of the way along the length of the leg. The legs of the nacelle support are arranged at equal angles around a vertical axis of the nacelle support. The legs of the nacelle support have the same lengths. The legs of the nacelle support are oriented at the same angles to a vertical axis of the nacelle support. Each of the legs of the nacelle support is attached at its lower end to a coupling structure. Each of the legs of the lower tower is attached at its upper end to the coupling structure. The legs of the lower tower are oriented at one angle to a vertical axis of the lower tower, and the legs of the nacelle support are oriented at a different angle to a vertical axis of the nacelle support. Each of the legs of the lower tower lies on a common plane with a corresponding one of the legs of the nacelle support.

The nacelle has a rotor shaft from which blades project and that rotates on a substantially horizontal axis of rotation. A free end of a rotor blade that is oriented vertically is at approximately the same vertical position as the waist and a horizontal distance from the free end of the rotor blade to the waist corresponds to the extent to which the free end is expected to flex or teeter. A nacelle support ring is attached to the upper end of the nacelle support. The nacelle support includes a yaw bearing for rotation of the nacelle about a vertical axis.

Each of the legs is constructed of angles, tubes, i-beams, or other structural elements. The waist is formed of tubes, tubular joints, welded plates, beams, or cast parts. The lower tower can be a monopole.

In general, in an aspect, a nacelle of a wind-driven electric generator is supported by a structure that has exactly three cylindrical tubular legs that define a lower tower that is relatively broad at its lower end and rises to a relatively narrow waist, and a nacelle support that is attached to the waist of the lower tower and supports the nacelle.

In general, in an aspect, a nacelle of a wind-driven electric generator is supported by a structure that includes a lower tower that is relatively broad at its lower end and rises to a relatively narrow waist, and a nacelle support that is attached at the waist of the lower tower and that supports the nacelle, in which the horizontal distance between a bearing point from which a blade rotor of the generator is supported and a central vertical axis of the superstructure is more than 5% of the vertical distance between the bearing point and the waist.

In general, in an aspect, a nacelle of a wind-driven electric generator is supported by a structure that includes at least three legs that define a lower tower that is relatively broad at its lower end and relatively narrow at its upper end, and in which the legs all are oriented at an angle to a vertical axis that is no less than 8 degrees and no more than 30 degrees.

In general, in an aspect, a nacelle of a wind-driven electric generator is supported by a structure that includes a nacelle support that is relatively narrow at its lower end where it is attached to the waist of the lower tower and relatively broader at its upper end where it supports the nacelle.

In general, in an aspect, a nacelle of a wind-driven electric generator is supported by a structure that includes three lower tower legs attached on one side of a connector to form at least a portion of a regular tetrahedron having a vertical central axis, and three nacelle support legs attached on an opposite side of the connector to form at least a portion of a second regular tetrahedron having the same vertical central axis.

In general, in an aspect, a structure to support a nacelle of a wind-driven electric generator is erected by using a crane that is shorter than the final height of the erected structure to help to raise at least a portion of the structure above a foundation to its final erected position. A temporary crane attached at or near the top of the erected structure may be used to lift at least some components of a nacelle, rotor shaft, rotor hub, and blades from the foundation.

These and other features and aspects, and combinations of them, can be expressed as methods, apparatus, systems, program products, means for performing a function, and in other ways.

Other aspects and features will become apparent from the following description and from the claims.

DESCRIPTION

FIG. 1 is a top perspective view of a support tower.

FIG. 2 is a side view of a support tower looking in direction A in FIG. 3.

FIG. 3 is a plan view of a support tower.

FIG. 4 is a side view of a support tower looking in direction B in FIG. 3.

FIGS. 5 through 10 are schematic perspective views of steps in erecting a support tower.

FIGS. 11 through 14 are schematic perspective views of steps in erecting a support tower.

As shown in FIGS. 1 through 4, in some implementations, a support tower 10 for a wind-driven electric generator (such as a wind turbine) 12 has a lower tower 14, a nacelle support 16, and a waist 18 between the lower tower and the nacelle support. By waist, we mean, for example, a location at which the support tower has its smallest cross-sectional area (as viewed from above). The wind turbine is shown schematically in FIG. 1 and only portions of it are shown in FIGS. 2 through 4, for clarity.

In the example shown in FIGS. 1 through 4, the lower tower is formed by three straight legs 20, 22, 24, each of which has a lower end attached to a stationary foundation coupling 26, 28, 30. The legs are symmetrically placed (that is, at equal angles Q, R, and S) about a vertical axis 13 of the lower tower. The three legs are inclined at a common angle 32 to vertical 13 and their upper ends 34, 35, 36, meet at the waist and impart a conical or partial tetrahedral shape to the lower tower.

The nacelle support 16 is formed as an inverted cone or partial tetrahedron by three straight legs 38, 40, 42. In the example shown in FIGS. 1 through 4, the three straight legs of the nacelle support are inclined at a common angle 33 to a vertical central axis of the nacelle support. In the example, the common angle 32 of the lower tower is different from the common angle 33 of the nacelle support, but the two common angles could be the same or differ by a greater amount than is shown in the figures. The three straight legs are arranged symmetrically about the vertical axis of the nacelle support and with opposite orientation of the symmetrical placement of the legs of the lower tower.

The lower ends of the legs of the nacelle support are connected to the waist and their upper ends are connected to a circular nacelle support ring 44 that lies in a horizontal plane.

In the example shown, the waist is in the form of six relatively short tubular sections 50, 52, 54, 56, 58, and 60. Each of the lower tower legs is connected to a pair of the legs of the nacelle support by a pair of the waist sections, as shown. In the resulting configuration, the legs of the nacelle support have an angular offset from the legs of the lower tower of 60 degrees as shown in FIG. 3.

The circular nacelle support ring 44 bears a stationary part 45 of a yaw bearing 46. The stationary part of the yaw bearing cooperates with a moving part 47 of the yaw bearing. A nacelle 13 is supported on the moving part 47. The moving part 47 refers, in the example shown, to four wheel assemblies 47 a, 47 b, 47 c (and a fourth one not shown) on which the nacelle is supported. The interface between the stationary part of the yaw bearing and the moving part 47 can be geared or not geared. The yaw bearing permits the nacelle to rotate about a vertical axis 13 so that a rotor axis 51 of the nacelle 75 has a desired relationship to (for example, is in line with) a direction 53 of the wind.

In some implementations, the support tower includes the lower tower, the nacelle support, the waist, the stationary part of the yaw bearing and any other parts that are stationary relative to the wind. The nacelle 75 includes all of the parts that move relative to the support tower, except for the rotor that is held on a shaft 55 of the nacelle. Among other things, the nacelle contains electric generators that are driven by the rotor as it is rotated by the wind, and equipment to rotate the nacelle about the vertical axis as the wind direction changes.

More information about the nacelle and its relationship to the nacelle support and the rotor, among other things, are contained in a provisional U.S. patent application, Ser. No. 61/043,327, and in a non-provisional patent application that claims the priority of the provisional application, is being filed on the same day as this application, and is titled “Wind-Driven Generation of Power.” Both the provisional application 61/043,327 and the “Wind-Driven Generation of Power” non-provisional patent application (hereinafter the “‘Wind-Driven Generation of Power’ patent applications”) are incorporated by reference here in their entirety. Combinations of one or more features described here with one or more features described in the “Wind-Driven Generation of Power” patent applications have advantages and may be claimed.

In some implementations, unlike what is shown in FIGS. 1 through 4, the three straight legs of the nacelle support may be collinear extensions of the three corresponding straight legs of the lower tower so that the support tower is formed in effect by three straight composite legs each extending from one of the foundation couplings to the platform.

In some implementations, each of the composite legs can be a monopole formed by a steel tube made up of a sequence of tubular sections. However, each of the legs could also be formed as a lattice or a combination of a tube and a lattice or in other configurations.

Among other advantages of this configuration of the support tower (and of a wide variety of other possible implementations) are one or more of the following.

The support tower can be very tall (and the corresponding position of the nacelle very high) to take advantage of better wind conditions at greater heights. At the same time, the span of the lower tower (defined, for example, by a circle 48 on which the lower ends of the three legs lie or by the area bounded by the triangle implied by the locations of the stationary foundation couplings) can be very large (and much larger than the typical 5 meter limit for the diameter of the largest section of an 80-meter monopole tower). The large diameter span at the base provides more resistance to wind-induced moments that would tend to overturn the support tower. For example, a tower that is 120 meters tall could have a height of 80 meters from the foundation to the waist and a height of 40 meters from the waist to the nacelle support ring.

In one example, with the legs of the tower support inclined 10 degrees to the vertical and a waist diameter of 9 meters (or a circle that includes the ends of the waist coupling pieces 50, 52, 54, 56, 58, and 60), each pair of legs at the base would be separated by 32 meters and the locations of the stationary foundation couplings would be on a circle having a diameter of about 37 meters. At the nacelle support ring, each pair of the legs of the nacelle support would be 17.25 meters apart and would lie on a circle having a diameter of about 19.9 meters, which would be the diameter of the nacelle support ring. In this configuration, a 40-meter long rotor blade 59 oriented vertically toward the ground would have a horizontal distance 61 of 7 meters from the outer edge of the waist. The resulting tower may be much stronger, stiffer and lighter weight than a monopole tower of the same height.

Because the waist of the support tower is relatively narrow, the vertical plane containing the rotor hub 66 can be located a relatively short horizontal distance 63 from the perimeter of the nacelle support ring. A clear space 64 defined by the waist, the lower tower, and the nacelle support accommodates the ends 62 of the blades of the rotor (and reduces the chances that a blade will strike the tower) even when bending in the wind or deliberate design or dynamic adjustment causes the ends of the blades to project inwardly toward the central axis of the tower. The blades also may be made more flexible than in typical designs to permit increasing aerodynamic damping in heavy wind gusts, to permit easier control of the turbine and to spill the wind in heavy gusts to reduce the chance of overloading the turbine. The support tower can be used in designs that are based on either upwind or downwind mounting of the rotor.

Because the nacelle support ring is much wider than is the case with a traditional monopole tower nacelle support, the drive train, including rotor shaft bearings, gears, and generators can be located on or inside the perimeter of the yaw bearing. Only a short length, e.g., a few meters, of the rotor shaft and rotor hub need extend beyond the perimeter of the yaw bearing. Because the distance between rotor hub and yaw bearing is short, the lever arm through which the rotor acts is short and certain parts of the nacelle can be made simpler and lighter than in some other designs.

The use of three large straight legs meeting at a waist achieves the reduced weight and broader lower tower advantages of a lattice structure while reducing (although in some examples not eliminating) the need for a small number of rigid horizontal and diagonal cross-sectional supports (i.e., braces) 68 like the ones used in power distribution line support towers. The visible spatial frequency of the support tower elements is lower than that of a lattice tower made with angled steel and thus more aesthetically pleasing to the eye.

The number and placement of horizontal and diagonal braces depends on other features of the design. In some implementations, horizontal braces between pairs of the legs can be placed half and three quarters of the distance from the bottom to the top of the lower tower as shown in FIG. 2 and half of the distance from the top to the bottom of the nacelle support. In some implementations, the bracing can be placed one-third of the distance from the bottom of the lower tower and one-third of the distance from the top of the nacelle support. In some implementations rigid diagonal braces can be placed between pairs of the legs, or between legs of the tower and the foundation or between the upper ends of the legs of the nacelle tower and the nacelle support ring, or combinations of those locations.

In addition to rigid braces, stabilizing cables 71 can be connected diagonally between pairs of points on the legs as shown in FIG. 2, and in other locations.

The rigid braces can be diagonal tubes that handle both compression and tension; the stabilizing cables are under tension only.

By using a small number of legs, the effective area of the support tower that faces the wind is less than for a monopole tower of the same height, which reduces aerodynamic drag on the support tower in high winds. In addition, because the legs of the nacelle support are spread apart at their upper ends, the horizontal distance between the blade tips and the waist can be many meters, and the sharp wind loading and unloading forces on a rotor blade as it passes the leg during rotation (sometimes called tower shadow) is reduced substantially compared to a monopole tower.

Because each leg is straight, it can be formed of similar straight sections that are each, say, 20 meters long (or up to, say, 40 meters long) and have a diameter that is no larger than 4 meters, and can be carried more easily and less expensively on trucks to the site without confronting highway overpass constraints that exist for larger diameter monopole towers.

The support tower can be designed so that, during erection, the sections of each leg can be assembled horizontally on the ground and lifted into position without requiring a tall, expensive overhead crane.

In one example of a sequence of erection steps, shown in FIGS. 5 through 10, the support tower is conceptually divided by a vertical plane through the waist. There are two major lifting steps and connections are made at the waist level. In FIG. 5, two legs of the lower tower (and cross braces) and a single leg of the nacelle support are assembled on the ground as a first piece 102.

In FIG. 6, a crane 104 lifts the piece from a point 106 on the waist to a height of 40 meters and an angle to the ground of 30 degrees. During the lifting, the piece 102 pivots about the foundation points 110, 112. In one example, this step could require a lifting force of 1800 kiloNewtons.

In FIG. 7, a force of 8500 kiloNewtons is applied on a cable 108 that is connected at point 106 and oriented at 20 degrees to the ground. In FIG. 8, the raised piece is stabilized by cables 114, 116.

In FIG. 9, a cable 122 is attached at one end 118 to a point 119 on a second piece 121, which has been assembled on the ground and attached at one end to the foundation support 113. The other end of the cable 122 is passed through point 106 and is pulled from a location 123 on the ground to cause the piece 121 to pivot about the foundation support and be raised into position where it can be connected to the piece 102 to form the support tower (FIG. 10). Once finished, a small crane (not shown) can be positioned at the top of the support tower on the nacelle support ring and used to lift the nacelle elements and other equipment to the top as needed.

In the example shown in FIGS. 11 through 14, the lower tower 14 is assembled on the ground with braces 131 and temporary braces 133 supported the third leg of the lower tower in the air. The crane 104 uses a force of, say, 1650 kiloNewtons to lift the waist end of the lower tower to a height of 40 meters. Then lifting is done by a cable 137 (9000 kiloNewtons) to pivot the lower tower about the two foundation supports 110 and 112 (FIG. 12) until it is in place (FIG. 13).

In FIG. 14, a central erection tower 140 is assembled and supported laterally at the waist. The nacelle support 142 is assembled on the ground and lifted into place to complete the support tower. Then a small crane can be positioned on the nacelle support ring as in the earlier described sequence.

Once the legs have been erected to form the tower, the nacelle support ring can hold a small construction crane for the purpose of lifting the turbine, including the nacelle and the rotor from the ground and into place on the tower. Lifting can be done stably because the broad spacing of the legs at the lower tower and the relatively narrower waist keep the parts of the nacelle, the blades, and other equipment being lifted, far away from the structural elements of the support tower, reducing the risk of bumping and damaging the support tower or blades.

Once the turbine has been lifted and attached, the power cables can be pulled easily through the legs from the lower tower to the nacelle.

The nacelle support ring 44 structurally connects the legs to reduce the chance of buckling of the support tower and legs and enables the support tower to better resist torsional loads. Diagonal braces between the legs and the nacelle support ring can also be provided to increase stability. The nacelle support and nacelle support ring could include extensible elements such as shock absorbers to dampen tower vibrations and reduce peak loads on the legs. The nacelle support ring can be formed of and braced by any typical beam shape including tubes, trusses, or I-beams.

A wide variety of alternative implementations of many aspects of the support tower are within the scope of the claims.

For example, the number of legs could be more than three, for example, four or five. The stationary foundation couplings could be arranged other than on a circle and could be arranged on a surface that is not planar and/or is not horizontal. Other supporting structures that might not be characterized as individual legs could be used, for example, lattices that form a conical lower tower or a conical nacelle support, or both, in a more continuous annular configuration than would be represented by discrete legs.

The cross-section of each leg need not be circular but could be another more complex shape that reduces weight while maintaining sufficient strength against forces imposed by the wind.

The contour of the lower tower or the nacelle support or both need not be a simple cone or portion of a tetrahedron but could be another shape (for example curved) that defines a relatively broad lower tower, a relatively broad top of the support tower, and a relatively narrower necked down region as a waist.

Each of the legs need not be made of straight sections, but could be curved.

The angle at which the conical (or other contour) lower tower meets the foundation or the waist can be within a wide range. Similarly, the angle of the nacelle support from the waist to the nacelle support ring can be within a wide range and not be the same as the angle from the base to the waist. The angles of the legs of the lower tower to the vertical and of the legs of the nacelle support to the vertical can be, for example, in the range of 8 degrees to 30 degrees.

To reduce the weight of the tower relative to its strength, each of the legs can have a ratio of outside diameter to steel thickness in a range between, for example, 50 and 170, or more specifically between 60 and 100.

Any configurations of legs, lower towers, nacelle supports, support tower connectors, and other elements of the support tower and nacelle could be used that provide one or more of a broad supporting lower tower, a high mounting location for the turbine, light weight, a small area confronting the wind, clearance for the ends of the rotor blades, reduced loading and unloading of the rotor as it turns, and easy transportability of sections of the structure, among other things.

The turbine rotor could have two blades, three blades, or more than three blades. By extending the rotor shaft in both directions from opposite ends of the nacelle, two rotors can be used, one upwind and one downwind.

Colors and color schemes may also be used to improve the aesthetic appearance of the support tower and nacelle. Some elements may be darker colors than others to cause the lighter colored elements to be less visible against the sky than the darker colored elements. For example, braces could be white and legs could be blue or grey. 

1. A structure to support a nacelle of a wind-driven electric generator comprising at least three legs that define a lower tower that is relatively broad at its lower end and rises to a relatively narrow waist, and a nacelle support that is relatively narrow at its lower end where it is attached to the waist and relatively broader at its upper end where it supports the nacelle.
 2. The structure of claim 1 in which each of the legs of the lower tower comprises a tubular structure.
 3. The structure of claim 2 in which each of the legs has a cylindrical cross-section along most of its length.
 4. The structure of claim 3 in which the ratio of the outside diameter of each of the legs to the thickness (D/t) of the leg is between 50 and 170
 5. The structure of claim 3 in which the ratio of the outside diameter of each of the legs to the thickness (D/t) of each of the legs is between 60 and
 100. 6. The structure of claim 1 in which each of the legs comprises an assembly of separately fabricated sections along its length.
 7. The structure of claim 6 in which each of the separately fabricated sections is no larger than 4 meters in one dimension and no larger than 40 meters in a second dimension.
 8. The structure of claim 1 that also includes one or more braces.
 9. The structure of claim 8 in which the braces are arranged horizontally or diagonally or both.
 10. The structure of claim 8 in which there are no more than six horizontal braces between any pair of the legs.
 11. The structure of claim 8 in which there is no more than three horizontal braces between any pair of legs.
 12. The structure of claim 8 in which the braces are attached at regular intervals along a leg.
 13. The structure of claim 8 in which at least one of the braces is attached to a leg at a point that is either about halfway or about one-third of the way along the length of the leg.
 14. The structure of claim 1 in which the legs are arranged at equal angles around a vertical axis of the lower tower.
 15. The structure of claim 1 in which the legs have the same lengths.
 16. The structure of claim 1 in which the legs are oriented at the same angles to a vertical axis of the lower tower.
 17. The structure of claim 1 in which each of the legs is attached at its lower end to a stationary foundation.
 18. The structure of claim 1 in which each of the legs is straight.
 19. The structure of claim 1 in which each of the legs is curved.
 20. The structure of claim 1 in which the nacelle support has at least three legs.
 21. The structure of claim 20 in which each of the legs of the nacelle support is tubular.
 22. The structure of claim 21 in which each of the legs of the nacelle support has a cylindrical cross-section along most of its length.
 23. The structure of claim 22 in which the ratio of the outside diameter of each of the legs to the thickness of the leg is between 50 and
 170. 24. The structure of claim 23 in which the ratio of the outside diameter of each of the legs to the thickness of the leg is between 60 and
 100. 25. The structure of claim 20 in which each of the legs of the nacelle support comprises an assembly of separately fabricated sections along its length.
 26. The structure of claim 25 in which each of the separately fabricated sections is no larger than 4 meters in one dimension and no larger than 40 meters in a second dimension.
 27. The structure of claim 1 in which the nacelle support also includes one or more braces.
 28. The structure of claim 27 in which the braces are horizontal or diagonal or both.
 29. The structure of claim 28 in which there are no more than six horizontal braces between any pair of the legs of the nacelle support.
 30. The structure of claim 28 in which there is no more than three horizontal braces between any pair of the legs of the nacelle support.
 31. The structure of claim 28 in which the braces are attached to a leg of the nacelle support at a regular intervals along the length of the leg.
 32. The structure of claim 28 in which at least one of the braces is attached to the leg about halfway or one-third of the way along the length of the leg.
 33. The structure of claim 1 in which the legs of the nacelle support are arranged at equal angles around a vertical axis of the nacelle support.
 34. The structure of claim 1 in which the legs of the nacelle support have the same lengths.
 35. The structure of claim 1 in which the legs of the nacelle support are oriented at the same angles to a vertical axis of the nacelle support.
 36. The structure of claim 1 in which each of the legs of the nacelle support is attached at its lower end to a coupling structure.
 37. The structure of claim 36 in which each of the legs of the lower tower is attached at its upper end to the coupling structure.
 38. The structure of claim 1 in which the legs of the lower tower are oriented at one angle to a vertical axis of the lower tower, and the legs of the nacelle support are oriented at a different angle to a vertical axis of the nacelle support.
 39. The structure of claim 38 in which each of the legs of the lower tower lies on a common plane with a corresponding one of the legs of the nacelle support.
 40. The structure of claim 39 in which each of the legs of the nacelle support has an angular offset of 60 degrees about a vertical axis of the structure relative to the orientation of a corresponding leg of the lower tower.
 41. The structure of claim 1 in which the nacelle has a rotor shaft from which blades project and that rotates on a substantially horizontal axis of rotation.
 42. The structure of claim 41 in which a free end of a rotor blade that is oriented vertically is at approximately the same vertical position as the waist and a horizontal distance from the free end of the rotor blade to the waist corresponds to the maximum extent to which the free end is expected to flex or teeter.
 43. The structure of claim 1 that also includes a nacelle support ring attached to the upper end of the nacelle support.
 44. The structure of claim 43 in which the nacelle support comprises a yaw bearing for rotation of the nacelle about a vertical axis.
 45. The structure of claim 1 in which each of the legs is constructed of angles, tubes, i-beams, or other structural elements.
 46. The structure of claim 1 in which the waist is formed of tubes, tubular joints, welded plates, beams, or cast parts.
 47. The structure claim 1 in which the lower tower comprises a monopole.
 48. The structure of claim 8 in which at least some of the braces are a lighter color than the legs.
 49. The structure of claim 48 in which the braces are white and the legs are blue or grey.
 50. A structure to support a nacelle of a wind-driven electric generator comprising exactly three cylindrical tubular legs that define a lower tower that is relatively broad at its lower end and rises to a relatively narrow waist, and a nacelle support that is attached to the waist of the lower tower and supports the nacelle.
 51. A structure to support a nacelle of a wind-driven electric generator comprising a lower tower that is relatively broad at its lower end and rises to a relatively narrow waist, and a nacelle support that is attached at the waist of the lower tower and that supports the nacelle, in which the horizontal distance between a bearing point from which a blade rotor of the generator is supported and a central vertical axis of the superstructure is more than 5% of the vertical distance between the bearing point and the waist.
 52. A structure to support a nacelle of a wind-driven electric generator comprising at least three legs that define a lower tower that is relatively broad at its lower end and relatively narrow at its upper end, and in which the legs all are oriented at an angle to a vertical axis that is no less than 8 degrees and no more than 30 degrees.
 53. A structure to support a nacelle of a wind-driven electric generator comprising a nacelle support that is relatively narrow at its lower end where it is attached to the waist of the lower tower and relatively broader at its upper end where it supports the nacelle.
 54. A structure to support a nacelle of a wind-driven electric generator comprising three lower tower legs attached on one side of a connector to form at least a portion of a regular tetrahedron having a vertical central axis, and three nacelle support legs attached on an opposite side of the connector to form at least a portion of a second regular tetrahedron having the same vertical central axis.
 55. A method for use in erecting a structure to support a nacelle of a wind-driven electric generator comprising using a crane that is shorter than the final height of the erected structure to help raise at least a portion of the structure above a foundation to its final erected position.
 56. The method of claim 55 also including using a temporary crane attached at or near the top of the erected structure to lift a nacelle and at least some components of a nacelle, rotor shaft, rotor hub, and blades from the foundation. 